/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) {
            return head;
        }

        auto p = head, q = p->next;
        while (q) {
            auto o = q->next;
            q->next = p;
            p = q, q = o;
        }
        head->next = NULL;

        return p;
    }
};
